Plasma processing apparatus and method

ABSTRACT

A plasma is generated by feeding an antenna with radio-frequency electric power generated by a radio-frequency power source, and one end of the antenna is grounded to the earth through a capacitor of variable capacitance. A Faraday shield is electrically isolated from the earth, and the capacitance of the variable capacitor is determined to be such a value that the voltage at the two ends of the antenna may be equal in absolute values and inverted to reduce the partial removal of the wall after the plasma ignition. At the time of igniting the plasma, the capacitance of the capacitor is adjusted to a larger or smaller value than that minimizing the damage of the wall.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a plasma processing apparatusfor surface treatment to etch a substrate or to form a thin film with aplasma by supplying a radio-frequency electric field to an antenna,generating an electric field, and thereby generating a plasma by theelectric field, and a method of using this apparatus. More particularly,the invention relates to a semiconductor processing apparatus forprocessing a semiconductor device, and a method of using this apparatus.

[0002] In a semiconductor processing apparatus for generating a plasmaby induction by feeding an electric current to a coil-shaped antenna,there is a problem that a vacuum chamber wall made of an non-conductivematerial and enclosing a plasma generating unit so as to establish avacuum atmosphere is partly removed by the plasma. In order to solvethis problem, there has been conceived a method using a field called the“Faraday shield”, as disclosed in Japanese Patent Laid-Open No.502971/1993. If the Faraday shield is used, however, the plasmaignitability is so deteriorated that the plasma is not ignited unless avoltage as high as tens of KV is applied to the feeding portion of thecoil-shaped antenna. This apparatus may fail with a high possibility bythe discharge between the antenna and a conductive structure nearby. Inorder to prevent this discharge, an additional structure is needed toinsulate the antenna from the existing structure, causing the apparatusto be complicated.

[0003] When a Faraday shield is used to reduce the partial removal ofthe wall, foreign matters are liable to adhere to the wall and to appearif its sticking rate to the wall from the plasma is accelerated.Therefore the partial removal of the wall must be adjusted according tothe process.

[0004] The plasma density distribution is determined mainly by thegeneration rate distribution and by the state of transportation of ionsand electrons. In the absence of an external magnetic field, thetransportation of the plasma diffuses isotropically in every direction.At this time, electrons instantly escape and tend to reach the wall ofthe vacuum chamber because the mass is no more than {fraction (1/1,000)}of that of an ion, but they are repelled by the sheath (ion sheath)formed in the vicinity of the wall. As a result, a quasi-neutralcondition of the electron and ion densities is always met in the plasma,so that both the ions and electrons are bipolarly diffused toward thewall. At this time, the potential of the plasma takes on its maximumwhere the plasma density, i.e., the ion density, is the maximum. Thispotential is termed the plasma potential Vp, approximately expressed byVp≈Te×ln(mi/me), where Te, mi and me are the electron temperature, themass of an ion, and the mass of an electron, respectively. In theplasma, the potential distribution is determined by the potential Vp andthe wall potential (ordinally at 0 V), so that the density distributionis correspondingly determined. Since, in this case, the plasma isconfined by the electrostatic field established by itself, the densitydistribution is determined by the shape of the apparatus, the placewhere the induced electric field takes on the maximum, and the ratio ofthe generation ratio/the bipolar diffusion flux.

[0005] When the coil is wound by several turns on the vacuum chamber,for example, the magnetic flux generated by the coil takes on themaximum at the central portion so that the induced electric field takeson the maximum at the central portion. Moreover, the induced electricfield cannot penetrate deeper than about the skin depth, e.g., 1 cm, sothat both the ionization factor and the dissociation factor take ontheir maximums at the radially central portion (in the direction ofarrow r, e.g., in FIG. 21(a)) and just below the dielectric member (inthe direction of arrow z, e.g., in FIG. 21(a)). After this, the plasmadiffuses towards the wafer side (downstream side). In the case of anordinary chamber having a cylindrical shape, therefore, the plasmadensity is the maximum at the central portion in the direction of arrowr, and the degree of central concentration rises downstream so that theplasma density becomes nonuniform in the region where the wafer isplaced.

SUMMARY OF THE INVENTION

[0006] A first object of the invention is to control the removal extentof the vacuum chamber wall around the plasma generating portion by theplasma. A second object of the invention is to improve the plasmaignitability.

[0007] A third object is to realize a uniform plasma of high density.This object is particularly desired in processing large semiconductorwafers (e.g., large-size semiconductor wafers of 300 mm).

[0008] In order to achieve the above-specified objects, according to theinvention, there is provided a plasma processing apparatus comprising anantenna (coil) for generating an electric field in a plasma generatingportion, a radio-frequency power source for supplying radio-frequencyelectric power to said antenna, a vacuum chamber enclosing the plasmagenerating portion to establish a vacuum atmosphere therein, a Faradayshield provided around said plasma generating portion (e.g., around thevacuum chamber), a gas supply unit for supplying gas into said vacuumchamber, a sample stage on which an object to be processed is placed,within the vacuum chamber, and a radio-frequency power source forapplying a radio-frequency electric field to said sample stage, a plasmabeing generated by accelerating electrons and ionizing them by collisionwith the electric field generated by said antenna, and therebyprocessing said object; characterized in that a load is provided in theearth portion of said antenna, the average potential of said antenna isadjusted so as to improve the ignitability at a plasma ignition time,and the load is adjusted after the plasma is produced so that theaverage potential of said antenna may be close to that of the earth, andthe removed amount of the wall of said vacuum chamber after the plasmageneration may be small. The above-specified objects are also achieved,according to the present invention, by a method of operation of thisapparatus whereby the load provided in the earth portion of the antennais adjusted (that is, the voltage on the ends of the antenna (coil) iscontrolled) such that ignitability of the plasma at the time of plasmaignition is facilitated, and is then again adjusted to be close to thatof the ground to limit the amount of chamber wall removed (e.g., etched)by the plasma.

[0009] Here, the phenomenon that the average potential of the antennacomes close to that of the earth means that the potentials 30 a and 30 bof FIG. 4 are mutually opposite in phase but substantially equal to eachother, that is, Va≈−Vb.

[0010] As another technique and structure to achieve the above-specifiedobjects, the Faraday shield can be provided with at least one switch.When igniting the plasma at a plasma ignition time, the at least oneswitch is positioned such that the Faraday shield is held in a floatingstate, to facilitate ignition of the plasma. Thereafter, the at leastone switch is thrown to ground the Faraday shield, so as to protect thewall of the plasma chamber from removal by the plasma.

[0011] As still another technique and structure to achieve theabove-specified objects, the load can be provided in the earth portionof the antenna and a switch or switches can be provided for the Faradayshield. By adjusting the load and positioning the switch as described inthe preceding paragraphs, ignition of the plasma is facilitated andremoval of the wall in the plasma chamber is avoided.

[0012] Means for solving the above-specified problems will be describedwith reference to FIG. 2. FIG. 2 shows an ordinary induction type plasmagenerating apparatus. With this apparatus, the methods for reducing thepartial removal of the vacuum chamber wall around the plasma generatingportion by the plasma and for improving the ignitability of the plasmaare examined by changing the way of grounding the Faraday shield and theantenna to the earth.

[0013] In this apparatus, a mixed gas of a chlorine gas and a borontrichloride gas is supplied into a vacuum chamber 2 made of alumina, bythe gas supply unit 4. The gas is ionized to produce a plasma 6 with theelectric field which is generated by a coil-shaped antenna 1 of twoturns wound around the vacuum chamber 2. After this plasma production,the gas is discharged to the outside of the vacuum chamber by adischarge unit 7. The electric field for producing the plasma isgenerated by feeding the antenna 1 with radio-frequency electric powerof 13.56 MHz generated by a radio-frequency power source 10. In order tosuppress the reflection of the electric power, an impedance matchingunit 3 is used to match the impedance of the antenna 1 with the outputimpedance of the radio-frequency power source 10. The impedance matchingunit is one using two capacitors of variable capacitance, generallycalled an “inverted L type”. The other end of the antenna is groundedthrough a capacitor 9 to the earth, and a switch 21 is provided forshorting the capacitor 9. In order to prevent the vacuum chamber 2 frombeing etched by the plasma 6, moreover, a Faraday shield 8 is interposedbetween the antenna 1 and the vacuum chamber 2. By turning on/off aplurality of switches 22, the Faraday shield can be brought into eitherthe grounded state or the ungrounded state. FIG. 3 is a perspective viewshowing the state that the Faraday shield is installed. This Faradayshield 8 is provided with a slit 14 for transmitting the inductiveelectric field 15 a generated by the coil-shaped antenna 1, into thevacuum chamber but intercepting a capacitive electric field 15 b. Theplasma is ignited mainly with the capacitive electric field 15 b. Whenthe Faraday shield is grounded to the earth, however, the capacitiveelectric field from the antenna is hardly transmitted into the vacuumchamber, thereby deteriorating the ignitability of the plasma. When theFaraday shield is not grounded to the earth, the antenna and the Faradayshield are capacitively coupled to bring the potential of the Faradayshield close to the average potential of the antenna. Thus, it isconsidered that the capacitive electric field is established between theFaraday shield 8 and an electrode 5, and hence the ignitability of theplasma is not deteriorated so much.

[0014] The capacitive electric field 15 b is normal to the wall of thevacuum chamber 2, so that the charged particles in the plasma areaccelerated to impinge upon and damage the wall. Light 16 emitted fromthe plasma was observed with a spectroscope 20, and the removal of thewall was measured by observing the light emission strength of aluminumin the plasma as the wall aluminum was removed.

[0015] First of all, here will be described a method for optimizing thecapacitance of the capacitor 9 connected to the earth portion of theantenna in the experimental apparatus shown in FIG. 2 so that theremoval of the wall may be reduced. In the following, the conductionstate between the two ends of the switch will be referred to as “on”,and the cut-off state will be referred to as “off”. With the switch 21being off, that is, with the capacitor 9 being not shorted, here will bedescribed the optimum value of the magnitude of the capacitance of thecapacitor 9. The experimental apparatus of FIG. 2 can be shown as anequivalent circuit in FIG. 4.

[0016] Then, the antenna 1 acts as the primary coil of a transformer,and the plasma 6 acts as the secondary coil of the same. The antenna 1and the plasma 6 are coupled capacitively, and their capacitance isshown by capacitors 31 a and 31 b. The capacitance C of the capacitor 9is determined so that a relation of Va=−Vb always holds between thepotential Va at the position of the point 30 a on the circuit and thepotential Vb at the position of the point 30 b when the antenna has aninductance L. When this condition is satisfied, the potentials to beapplied to the two ends of the capacitors 31 a and 31 b are minimized,minimizing the wall damage. FIG. 5 further simplifies the circuit ofFIG. 4, namely the antenna and the plasma are combined together as anelement 17 having one combined impedance. The impedance of the elementwas experimentally determined to be Z1=2.4+141j(Ω), where j is a complexnumber. This measurement of the impedance can be simply executed bymeasuring the electric current flowing through an object to be measured,and the voltages at the two ends of the object. The capacitor 9 has animpedance Z2=−(1−ωC)j , where ω is the angular frequency correspondingto 13.56 MHz. For Va=−Vb, the relation between the impedances Z1, Z2 is(Z1+Z2):Z2=1:−1 since the real part of Z1 is so small that it can beignored. The calculated electric capacitance of the capacitor 9 is about150 pF, therefore, the relation Va=−Vb holds. FIG. 6 illustrates theresults of calculation of the amplitudes of the potentials at the point30 a (the dotted curve) and the point 30 b (the solid curve). The graphshows the capacitance of the capacitor 9 as the abscissa, and theamplitudes of the generated potentials as the ordinates. As a result,the generated potentials were mutually equal in the vicinity of thecapacitance of 150 pF of the capacitor 9, the phases of the oscillatingvoltages at that time were shifted by 180 degrees, and the relation ofVa=−Vb was satisfied. This makes it possible to determine by the methodthus far described such a capacitance of the capacitor to be connectedto the earth side of the antenna that the damage of the wall isminimized.

[0017] Next, with the capacitance of the capacitor 9 fixed at 150 pF inFIG. 2, the removed amount of the wall and the plasma ignitability wereexamined, as tabulated in FIG. 15, when the switches 21 and 22 areturned on or off. The wall removal is found to be great when the switch21 is on and the switch 22 is off. Under this condition, the plasmaignitability is excellent. Under the other conditions, however, the wallremoval can be reduced, but the plasma ignitability is low. Therefore,it has been found that the condition for little wall removal and forexcellent plasma ignitability is not present in this system. However,these two purposes can be achieved by operating either the switch 21 orthe switch 22 so as to reduce the wall damage after the plasma wasignited under the condition that the switch 21 is on and the switch 22is off at the ignition time. Here, it is better to use only the switch21 for the simplification of the apparatus structure. This is partlybecause the potential of the Faraday shield has to be lowered to zero asmuch as possible so as to reduce the wall damage by using the switch 22,and consequently the switch 22 has to be provided in plurality, andpartly because the Faraday shield has to be grounded with the shortestdistance to the earth so that the plural switches 22 have to be providedjust near the antenna and the Faraday shield. If the plural switches arearranged for those necessities at the portion adjacent to the antennaand the Faraday shield, the result is a complicated structure. Thiscomplicated structure can be avoided with respect to the switch 21because only one switch 21 is connected to the capacitor 9 side which isprovided at a considerable distance from the antenna.

[0018] The off state of the switch 21 is the state that the capacitor of150 pF is connected between the antenna and the earth, and the on stateof the switch 21 is identical to the state that the capacitance of thecapacitor 9 is increased to infinity in a radio-frequency band of HF orVHF. This means that the wall removal increases more as the capacitanceof the capacitor 9 is raised to a higher level from 150 pF. The wallremoval also increases even if the capacitance of the capacitor 9 islowered from 150 pF. Thus, the wall removal can be controlled by varyingthe capacitance of the capacitor 9.

[0019] In an apparatus shown in FIG. 7, the capacitance of the capacitor9 connected to the earth side of the antenna 1 is variable, so that thewall removal by the plasma can be reduced by varying the capacitance ofthe capacitor 9. Moreover, the plasma ignitability can be drasticallyimproved by making the capacitance of the capacitor 9 far larger orsmaller than 150 pF at the time of igniting the plasma.

[0020] By adjusting the capacitance of the capacitor connected to theearth side of the antenna, as described above, the removed amount of thewall by the plasma can be reduced to achieve the first object of theinvention. At the plasma ignition time, moreover, the capacitance of thecapacitor connected to the earth side of the antenna can be changed toestablish an excellent ignitable state, thereby achieving the secondobject of the invention.

[0021] Here will be examined a method for generating a uniform plasma.When the coil-shaped antenna is placed on the upper face of the vacuumchamber, the induced electric field is generated at the central portion,even if the diameter of the antenna is varied to vary the intensity ofthe induced electric field in the radial direction, so that the plasmadensity distribution is nonuniformly concentrated at the center. Thistendency of concentration of the plasma density at the center is notvaried even if a plurality of antennas are arranged to vary the distancebetween each antenna and the dielectric member. FIG. 21(b) illustratesone example of the calculation of the plasma density distribution whenthe antenna is placed on the vacuum chamber like FIG. 21(a). From thiscalculation, when the ratio of the apparatus height H to the radius R(the aspect ratio) is as large as H/R=20/25, as illustrated in FIG.21(b), the plasma density at the place, just below the antenna (z=2 cm),where the antenna is present takes on its maximum and increases in itsabsolute value (z=10 cm) downstream (in the direction where the value zincreases) but is small just above the substrate. It is then found thatthe plasma density is nonuniform. When viewed in the z direction, thedensity takes on its maximum at the apparatus center z=10 cm. When theaspect ratio is reduced, as illustrated in FIG. 21(c), the densitydistribution is substantially identical to that of FIG. 21(b), but thedistribution just above the substrate is gentler than that of (b) and isconcentrated at the center.

[0022] The plasma density distribution is determined by the boundarycondition that the plasma density is zero on the vacuum chamber wall andby the generation rate distribution, i.e., the antenna position. Even ifthe antenna position is changed, as illustrated in FIG. 21(d ), and if aplurality of antennas are placed to change the power distribution, theshape of the density distribution remains unchanged. When the coil isprovided on the upper face, the induced electric field generated by theantenna takes on its maximum just below the antenna, so that a centrallyconcentrated distribution is always established on the downstream.

[0023] In the case of an arrangement in which the antenna is woundhorizontally around the vacuum chamber, the induced electric field takeson its maximum on the side face of the chamber. A sheath is formed onthe side face of the chamber, so that the plasma density takes on itsmaximum slightly inside the sheath, at the place the closest to theantenna. As shown in a horizontal section at this time, the potential ishigher at the sheath end than at the wall and than at the plasma centerso that the plasma is transported to the two sides from the sheath.Simultaneously with this, the plasma flows downstream from thatposition, and hence the density distribution is uniform in a portion ina horizontal section, at a distance in the z direction from the highestdensity portion. In the case of a cylindrical apparatus, for example, aconcave distribution may be established in the vicinity of the wafer fora small H/R ratio, and a convex distribution may be established for asufficiently large H/R ratio where H is the height of the apparatus andR is the radius thereof, so that the plasma density distribution can becontrolled to some extent (refer to FIGS. 22(a) and 22(b)). The dominantfactors at this time are the shape of the apparatus, i.e., the ratioH/R. When the antenna is provided on the side face, however, the plasmadensity is lowered by the reduction in the coupling efficiency due tothe large coupling area of the antenna and the plasma and by the largeloss of the plasma because the region where the density is a maximum isnear to the side face wall. If the supplied power and the vacuum chambersize are the same, the plasma density of this case is lower than that ofthe aforementioned case in which the antenna is provided on the upperface. This raises a problem that the processing speed of the object tobe processed is low.

[0024] As thus far described, the plasma density distribution of theinductively coupled plasma varies with the apparatus shape and theantenna arrangement, but the third object of the invention is achievedby such a construction where the upper face of the vacuum chamber has asmaller area than that of the lower face, and the upper face is flat.Thus, the apparatus of the present invention can be used to processlarge-sized semiconductor wafers discussed previously.

[0025] In the plasma processing apparatus, preferably, the angle betweenthe edge at which the lower face and the upper face intersect and thenormal of the upper face is not less than 5 degrees.

[0026] In the plasma processing apparatus, more preferably, the ratio ofthe apparatus height (the distance from the object to be processed tothe upper face) to the radius is not more than 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 shows a structure of a first embodiment of the invention.

[0028]FIG. 2 shows a structure of an experimental system used forverifying the invention.

[0029]FIG. 3 shows the state that a Faraday shield is mounted.

[0030]FIG. 4 shows an equivalent circuit diagram of the experimentalsystem used for verifying the invention.

[0031]FIG. 5 shows an equivalent circuit diagram of an experimentalsystem used for verifying the invention.

[0032]FIG. 6 shows a graph illustrating the amplitude of a potentialestablished between the two ends of an antenna.

[0033]FIG. 7 shows a diagram of an experimental system used forverifying the invention.

[0034]FIG. 8 shows a structure of a second embodiment of the invention.

[0035]FIG. 9 shows a structure of a third embodiment of the invention.

[0036]FIG. 10 shows a fourth embodiment of the invention.

[0037]FIG. 11 shows a structure of a fifth embodiment of the invention.

[0038]FIG. 12 shows a structure of a sixth embodiment of the invention.

[0039]FIG. 13 shows a structure of a seventh embodiment of theinvention.

[0040]FIG. 14 shows a perspective view of a plasma processing apparatusindicating the flow of eddy current in the seventh embodiment of theinvention.

[0041]FIG. 15 shows a table indicating the switches 21 and 22, theremoved amount of the wall of a vacuum chamber and RF powers necessaryto ignite a plasma.

[0042]FIG. 16 shows a plasma processing apparatus of an eighthembodiment of the invention.

[0043] FIGS. 17(a) and 17(b) show a plasma processing apparatus of aninth embodiment of the invention.

[0044]FIG. 18 shows a plasma processing apparatus of a tenth embodimentof the invention.

[0045]FIG. 19 shows a plasma processing apparatus of an eleventhembodiment of the invention.

[0046]FIG. 20 shows a plasma processing apparatus of a twelfthembodiment of the invention.

[0047] FIGS. 21(b)-21(d) show the plasma density distribution when theantenna is placed on the upper face of the plasma processing apparatusas shown in FIG. 21(a).

[0048]FIG. 22(b) shows the distribution of ion current incident on thewafer when the antenna is placed on the side face of the plasmaprocessing apparatus, as shown in FIG. 22(a).

[0049] FIGS. 23(a) and 23(b) show schematic diagrams illustrating aprinciple of the invention.

[0050] FIGS. 24(b) and 24(c) show diagrams illustrating the distributionof ion current incident on the wafer in the case of the invention,wherein the apparatus is schematically illustrated in FIG. 24(a).

[0051]FIG. 25 a diagram illustrating the effects of the fourthembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0052] Embodiments of the present invention will be described in thefollowing. These embodiments are only illustrative of the presentinvention, the present invention being defined by the appended claims.

[0053]FIG. 1 shows a first embodiment of the semiconductor processingapparatus according to the present invention. In the present apparatus,a material gas of, e.g., oxygen, chlorine, boron trichloride or the liketo be used for processing a semiconductor is supplied into a vacuumchamber by a gas supply unit 4 and is ionized to generate a plasma 6with the electric field which is generated by a coil-shaped antenna 1.After this plasma generation, the gas is discharged to the outside ofthe vacuum chamber by a discharge unit 7. The plasma generating electricfield is generated by supplying the antenna 1 with radio-frequencyelectric power which is generated by a radio-frequency power source 10of 13.56 MHz, 27.12 MHz, 40.68 MHz or the like. In order to suppress thereflection of the electric power, however, an impedance matching unit 3is employed to match the impedance of the antenna 1 with the outputimpedance of the radio-frequency power source 10. The impedance matchingunit is of a so-called inverted L type. Depending on the frequency orthe structure of the antenna, however, it is necessary to employ animpedance matching unit by which matching is easy. The other end of theantenna 1 is grounded to the earth through a capacitor 9 having avariable capacitance. Between the antenna 1 and the vacuum chamber 2,there is interposed a Faraday shield 8 for preventing the vacuum chamber2 from being adversely etched by the plasma 6. The Faraday shield is notelectrically grounded. As shown in FIG. 3, moreover, the Faraday shield8 has a slit perpendicular to the direction in which the coil of theantenna is wound. A semiconductor wafer 13 to be processed is placed onan electrode 5. In order to attract ions existing in the plasma to thespace above the wafer 13, an oscillatory voltage is applied to theelectrode 5 by a radio-frequency power source 12. It is important thatthe capacitance of the variable capacitor 9 be a value at which thepartial removal of the wall is minimized.

[0054] In the present embodiment, at the time of igniting the plasma 6,the capacitance of the capacitor 9 is adjusted to a value larger orsmaller than that which minimizes the wall removal. The capacitance isadjusted to about two times or one half as large as the value whichminimizes the wall removal, so that the plasma can be ignited with theradio-frequency power of several tens of watts.

[0055] After this plasma ignition, the capacitance of the capacitor 9 isbrought closer to the value minimizing the damage so that the scrapingof the wall may be minimized. When the wall should be partly removed tosome extent from the standpoint of a foreign substance, the capacitanceof the capacitor 9 is determined to be a value to cause desired removal.The optimum value has to be determined by repeating the semiconductorprocess.

[0056] A second embodiment of the invention will be described withreference to FIG. 8. The basic construction of the apparatus of thepresent embodiment is identical to that of the first embodiment, butwhat is different from the first embodiment is the structure of thecapacitor provided at the earth side of the antenna 1. In the presentembodiment, two capacitors, a capacitor 9 a and a capacitor 9 b, areconnected in parallel with the earth side of the antenna 1. Of these,the capacitor 9 a is connected directly to the earth whereas thecapacitor 9 b is connected through a switch 21 to the earth.

[0057] When the capacitance of the capacitor 9 a is adjusted to thevalue to minimize the damage, the capacitance provided to the earth sideof the antenna 1 is increased by the amount corresponding to thecapacitance of the capacitor 9 b by turning on the switch 21 at the timeof igniting the plasma, so that the plasma ignitability is improved bymaking the capacitance of the capacitor 9 b sufficiently high. After theplasma ignition, the switch 21 is turned off to minimize the removal ofthe wall. If the removal is desired to some extent from the standpointof foreign substance, as in the first embodiment, the capacitance of thecapacitor 9 a may be adjusted to a value for the desired wallplasma-etching.

[0058] A third embodiment of the invention will be described withreference to FIG. 9. The basic construction of the apparatus of thepresent embodiment is identical to that of the second embodiment, butwhat is different from the second embodiment is the employment of aninductor 19 in place of the capacitor of FIG. 8. When the capacitor 9has an capacitance C, the inductor 19 has an inductance L and theradio-frequency power source 10 outputs radio-frequency waves having anangular frequency ω, the impedance Z between the earth side of theantenna and the earth is expressed by Z=−(1/ωC)j, when the switch 21 isoff, and by Z=−(1/( ωC−1/ωL))j when the switch 21 is on. When thecapacitance of the capacitor 9 is adjusted to minimize the wall removal,with the switch 21 off, the value Z can be changed by operating theswitch 21 to improve the ignitability of the plasma. At the time ofigniting the plasma, therefore, the switch 21 is turned on to ignite theplasma. After the plasma ignition, the switch 21 is turned off tominimize the wall damage. If the wall should be partly removed to someextent from the standpoint of foreign matter, the capacitor 9 may be setto the value for the desired wall scraping.

[0059] In the third embodiment, there has been described a method ofvarying the impedance of the load connected between the antenna and theearth, by combining the capacitor, the inductor and the switch. By usingmeans other than that of this embodiment for varying the value of theimpedance of the load, it is possible to establish a state that anexcellent plasma ignitability is achieved and a state that a smallerwall removal is achieved.

[0060] A fourth embodiment of the invention will be described withreference to FIG. 10. The basic construction of the apparatus of thepresent embodiment is identical to those of the first, second and thirdembodiments, but the difference of the present embodiment is that theFaraday shield 8 made of a conductive material is buried in the wall ofthe vacuum chamber 2 made of a non-conductive material. As the materialfor the vacuum chamber 2, there is used alumina or glass. Since a metalsuch as chromium or aluminum can be easily fused to the alumina, apattern thereof can be formed in the aluminum. When glass is used, ametal foil can be buried in the glass as in the defrosting heater of anautomobile.

[0061] The advantages as obtained from the structure in which theFaraday shield 8 is buried in the wall of the vacuum chamber 2, are thatthe insulating structure can be eliminated from between the antenna andthe Faraday shield 8, and that the distance between the vacuum chamber 2and the antenna 1 can be reduced to make the apparatus compact.

[0062] A fifth embodiment of the invention will be described withreference to FIG. 11. The basic construction of the apparatus of thepresent embodiment is identical to that of the fourth embodiment, butthe difference of the present embodiment is that a wall surface of thevacuum chamber 2 made of a non-conductive material is covered with afilm made of a conductive material and acting as the Faraday shield. Inthe present embodiment, as an example, the internal side of the vacuumchamber on the plasma side is coated with the conductive Faraday shield8, but similar effects can be attained even when the atmospheric side ofthe vacuum chamber is coated with the Faraday shield 8.

[0063] In the present embodiment, the plasma 6 comes in direct contactwith the Faraday shield 8, and therefore the wall of the vacuum chamber2 is partly removed by the plasma 6 at the slit portions of the Faradayshield 8. In an oxide film etching process using oxygen as the materialgas, although depending upon the process, excellent fusibility betweenalumina and aluminum is utilized, realizing a construction in which aninsulating material is coated with a conductive material, by using aFaraday shield 8 of conductive aluminum and a vacuum chamber 2 ofinsulating aluminum (e.g., alumina). In the case of the metal process inwhich the material gas is chlorine or boron trichloride, the purpose canbe achieved by adopting alumina as the insulating material and SiC asthe conductive material. Many other combinations can be conceived, andsimilar effects can be expected from any combination if the combinationbrings about such performances that the coating conductive material ishardly removed even if the temperature of the vacuum chamber rises, andthat both the insulating material and the conductive material are hardlyremoved by the plasma.

[0064] A sixth embodiment of the invention will be described withreference to FIG. 12. The basic construction of the apparatus of thepresent embodiment is identical to those of the first, second and thirdembodiments, but what is different from those embodiments is that theFaraday shield 8 is grounded to the earth through a resistor 18.

[0065] It is expected that a worker may frequently touch the Faradayshield 8 at the time of reassembling the apparatus. A mechanism isrequired for preventing the Faraday shield from being charged at thattime. In the present embodiment, a resistor 18 is used to ground theFaraday shield to the earth. The resistance of this resistor 18 has tobe a higher impedance than that of the capacitance between the Faradayshield 8 and the earth, at the frequency of the radio-frequency powersource 10 for generating the plasma. For this necessity, if the groundedresistor 18 has a resistance R and if the radio-frequency waves to beoutputted by the radio-frequency power source 10 have an angularfrequency ω, the resistance R should satisfy R>1/ωC. In other words, theFaraday shield and the earth are coupled to the load to give a higherimpedance than that of the capacitance between the Faraday shield andthe earth, at the high frequency for generating the plasma, and theimpedance of the load is low in direct current, thereby preventing theFaraday shield from being charged at the end of the operation.

[0066] A seventh embodiment of the invention will be described withreference to FIG. 13. The basic construction of the apparatus of thepresent embodiment is identical to that of the sixth embodiment, butwhat is different from the sixth embodiment is that the vacuum chamber 2is made of a conductive material to produce the effect of the Faradayshield.

[0067] Since the vacuum chamber also acting as the Faraday shield cannotbe provided with a slit to shut off the inductive electric field, as hasbeen described with reference to FIG. 3, the inductive electric fieldhas to be able to pass by adjusting the thickness of the wall of theconductive vacuum chamber. Here will be disclosed a structure in whichthe vacuum chamber is electrically floated from the earth by aninsulating flange 24.

[0068] In the case of the present embodiment, no work of providing aFaraday shield around the vacuum chamber is required, improving theworkability. In the present embodiment, the circuit for adjusting theaverage potential to a larger absolute value than that at the vicinityof the earth or of the earth itself is identical to that of the sixthembodiment.

[0069]FIG. 14 is a perspective view showing the behavior of eddy currentflowing in the vacuum chamber in the present embodiment. The eddycurrent for preventing an inductive electric field 15 a, as describedwith reference to FIG. 3, from being transmitted into the vacuum chamber2 will flow in the circumferential direction indicated by arrow 25, ofthe vacuum chamber 2 having a cylindrical shape. If a relation R>ωLholds among the resistance R, the inductance L in the path of the eddycurrent and the angular frequency ω of the radio-frequency wavesoutputted from the radio-frequency power source 10, the eddy currentattenuation by the resistor is increased and hence the inductiveelectric field is transmitted into the vacuum chamber.

[0070] The vacuum chamber 2 has to be made of such a material that it ishardly removed by the plasma, because it is directly exposed to theplasma as in the fifth embodiment. Since the vacuum chamber ordinarilyhas a wall as thick as about 2 cm, it may be made of a material havingan electrical resistivity of about 0.02 Ωm so as to achieve such a skinthickness at a frequency of 13.56 MHz.

[0071] The vacuum chamber 2 is insulated from the earth by using theinsulating flange 24 and is equipped with the charge preventing resistor18 as in the sixth embodiment. The resistance of the resistor 18 has tobe a higher impedance than that between the Faraday shield and the earthat the frequency of the radio-frequency power source 10 for generatingthe plasma. In the semiconductor processing, a bias voltage is appliedto the electrode 5 by the radio-frequency power source 12. If the plasmais electrically floated from the earth, however, a high bias voltage isnot generated between the plasma and the electrode. In order to preventthis, the plasma has to be brought as much as possible into contact withthe earth thereby to lower the potential of the plasma. This lowering ofthe potential of the plasma can be achieved by allowing the resistanceof the resistor 18 to have a lower impedance than that between theFaraday shield and the earth in the frequency band of theradio-frequency power source 12.

[0072] The present embodiment is directed to an apparatus in which thevacuum chamber is wholly made of the conductive material. Similareffects could be achieved by eliminating the slits from the Faradayshields of the foregoing embodiments and by adjusting the thickness ofthe conductive material as in the present embodiment.

[0073] The foregoing embodiments described comprise the vacuum chamber 2having a cylindrical shape. Even if the vacuum chamber 2 is given anincline side face, has a trapezoidal section and is equipped with a coiland a Faraday shield, such a vacuum chamber 2 can be used in foregoingembodiments likewise.

[0074] An eighth embodiment of the invention will be described withreference to FIG. 16. The basic apparatus construction of the presentembodiment is identical to those of the first, second and thirdembodiments. What is different from the other embodiments is that thearea of the upper face 2 a (far from the electrode 5 for the object tobe processed) of the vacuum chamber is smaller than that of the lowerface. Preferably, the upper face is flat. In the invention thusconstructed, the degree and position of the coupling of the plasma andthe antenna can be varied according to the arrangement of the antenna,the number of turns of the antenna, the distance between the antenna andthe vacuum chamber, and so on. When the number of turns of the coil ofthe antenna is one the antenna is installed horizontally, for example,the coupling position is varied as the antenna is moved vertically, asillustrated in FIG. 23(a). When the number of turns of the coil is morethan one, the coupling state can be varied (FIG. 23(b)) according to thevertical position of the antenna and the distance between each of theturns and the vacuum chamber. The antenna may be moved upward, when thedensity at the central portion is increased, and downward when thedensity is intended to have a distribution where the density is high atthe periphery. Thus, the coupling position can be varied because theapparatus shape is given a gradient by the large lower face area and thesmall upper face area. In the case of an inductive coupling plasma, theelectrons/ions are isotropically diffused towards the chamber wall bythe bipolar diffusion so that their distributions are influenced by thechamber shape. As a result, the plasma distribution is easily flattenedif the upper face is flat. The control of the plasma densitydistribution is facilitated by the antenna arrangement and thecharacteristic apparatus shape. Because of the static field by theantenna 1, moreover, many foreign matters and reaction products areproduced in the vicinity of the antenna by the interaction between theplasma and the vacuum chamber wall 2. Because of the large area of thelower face, however, a path is formed along the chamber wall and thedischarge line 7 to allow the gas to flow easily along the wall, so thatthe rate of flow towards the wafer 13 to be processed can be reduced torealize a satisfactory processing.

[0075] A ninth embodiment of the invention is shown in FIG. 17. Thebasic apparatus construction of the present embodiment is identical tothat of the eighth embodiment. What is different from the otherembodiments is that the angle (see FIG. 18) between the edge where theupper face 2 a of the vacuum chamber 2 and the lower face 2 b thereofintersect (that is, the side of the vacuum chamber 2) and the normal ofthe upper face is not less than 5 degrees. FIG. 24 shows thedistribution of the density of the ion current incident on the surfaceof the object when the shape of the vacuum chamber is such that, forexample, the ratio of the upper surface radius Ru to the lower faceradius Rd is 4:5. For the vacuum chamber height H=13 cm, the ion currentis flat up to φ=300 (r=15 cm). If the height H is increased, thedistribution is shown by a curve the center of which is rather high. Ithas also been confirmed that the curve is high at the periphery when theheight H is decreased. If tan⁻¹{(Rd−Ru)/H}≧5 degrees, it is possible torealize the distributions which are flat and higher at the centralportion and at the periphery.

[0076]FIG. 18 shows a tenth embodiment of the invention. The basicapparatus construction of the present embodiment is identical to that ofthe eighth embodiment, but what is different from the other embodimentsis that the ratio H/R of the height H (i.e., the distance from theelectrode 5 to the upper face 2 a) of the vacuum chamber 2 to the radiusR of the vacuum chamber 2 is H/R≦1. This relation is satisfied, forexample, by the shape of the vacuum chamber of FIG. 24(a).

[0077]FIG. 19 shows an eleventh embodiment of the invention. The basicapparatus construction of the present embodiment is identical to that ofthe eighth embodiment, but what is different from the other embodimentsis that magnetic field generating means 16 is disposed outside thevacuum chamber 1. The plasma density distribution just above thesubstrate in the presence of the magnetic field is illustrated in FIG.25. From the graph showing the plasma density distribution, it is foundthat the plasma density is higher in the periphery as the magnetic fieldis increased. Thus, the magnetic field generating means acts as anauxiliary one capable of controlling the distribution.

[0078]FIG. 20 shows a twelfth embodiment of the invention. The basicapparatus construction of the present embodiment is identical to that ofthe eighth embodiment, but what is different from the other embodimentis that a plate 27 made of a conductor or a semiconductor is placed onthe face confronting the electrode 5 or on the inner side of the upperface 2 a of the vacuum chamber. Moreover, radio-frequency voltageapplying means 28 is preferably connected with the plate 27 to applyradio-frequency waves. Instead of the radio-frequency waves a pulsatingDC voltage may be used. Alternatively, the plate 27 may be grounded tothe earth.

[0079] By employing the present embodiment, the partial removal of thevacuum chamber wall enclosing the plasma generating portion by theplasma can be controlled while improving the plasma ignitability.

[0080] Moreover, by varying the degree and position of the coupling ofthe plasma and the antenna according to the arrangement of the antenna,the number of turns of the coil of the antenna, the distance between theantenna and the vacuum chamber and so on, the plasma distribution can becontrolled to establish a uniform plasma.

[0081] Many different embodiments of the present invention may beconstructed without departing from the spirit and scope of theinvention. It should be understood that the present invention is notlimited to the specific embodiments described in this specification. Tothe contrary, the present invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the claims.

What is claimed is:
 1. A plasma processing apparatus comprising: anantenna for generating an electric field in a plasma generating portion;a radio-frequency power source for supplying radio-frequency electricpower to said antenna; a vacuum chamber enclosing the plasma generatingportion to establish a vacuum therein; a Faraday shield provided aroundsaid plasma generating portion; a gas supply unit for supplying gas intosaid vacuum chamber; a sample stage on which an object to be processedis placed; and a radio-frequency power source for applying aradio-frequency electric field to said sample stage, such that a plasmais generated by accelerating electrons and ionizing the gas by collisionwith the electric field generated by said antenna, so as to process saidobject, wherein the vacuum chamber has an upper face and a lower face,and wherein the upper face of said vacuum chamber has a smaller areathan that of the lower face, and the upper face is flat.
 2. A plasmaprocessing apparatus according to claim 1 , wherein the upper face ofthe vacuum chamber is circular in plan view, the vacuum chamber has sidefaces joining the upper and lower faces, and an angle contained betweenthe side faces joining the lower face and the upper face and the normalto the upper face is not less than 5 degrees.
 3. A plasma processingapparatus according to claim 1 , wherein a ratio of (a) a distance fromthe object to be processed to the upper face, to (b) a radius of theupper face of the vacuum chamber, is not more than
 1. 4. A plasmaprocessing apparatus according to claim 1 , wherein a magnetic fieldgenerating means is provided outside the vacuum chamber.
 5. A plasmaprocessing apparatus according to claim 1 , wherein a plate made of aconductor or a semiconductor is placed on an inner side of the upperface of the vacuum chamber.
 6. A plasma processing apparatus accordingto claim 5 , wherein a radio-frequency power source is applied to saidplate so as to apply radio-frequency waves to said plate.
 7. A plasmaprocessing apparatus according to claim 5 , wherein a DC voltage sourceis applied to said plate so as to supply DC voltage to said plate.
 8. Aplasma processing apparatus according to claim 5 , wherein said plate isgrounded.
 9. A plasma processing apparatus according to claim 1 ,wherein the upper face of the vacuum chamber is flat, and wherein theupper face of said vacuum chamber has a smaller area than that of thelower face.